1. Field of the Invention
The present invention relates to a truing method and apparatus for truing a grinding surface of a grinding wheel with a truing roll being rotationally driven.
2. Discussion of the Related Art
Japanese Patent No. 2749154 describes a truing apparatus for a grinding machine in which a rotating grinding wheel and a rotating workpiece are relatively moved to grind the workpiece with the grinding wheel. In the grinding machine, a work spindle is rotatably supported on a work head for supporting the workpiece, a chuck device for gripping the workpiece and a truing roll for truing the grinding wheel are in turn fixed on an extreme end of the work spindle in alignment, and the grinding wheel and the work head are moved relatively in two directions orthogonal to each other to true the grinding surface of the grinding wheel with the truing roll.
In a recent grinding machine equipped with a grinding wheel using CBN (Cubic Boron Nitride) abrasive grain, the grinding wheel is rotated at a high speed so that the circumferential speed of the grinding wheel is increased to enhance the grinding efficiency. The ratio in circumferential speed of the grinding wheel to the truing roll is set conventionally in a range of 0.75 to 0.8 in order to true the grinding wheel to be sharp. For example, where the circumferential speed of the grinding wheel is set to 120 m/s (meter per second), the rotational speed of the truing roll would be set to a range of 15,000 to 20,000 min−1 (revolutions per minute) because the diameter of the truing roll is 100 mm (millimeters) or so. This would require that the truing roll be mounted on a rotational spindle which is able to be rotated at an extremely high speed.
Since the work spindle of the work head is not able to be rotated at such a high speed, it is practiced in place of the truing apparatus described in the aforementioned Japanese patent that a rotational spindle rotatable by a built-in motor at a high speed is supported on an apparatus main body and that a truing apparatus with a truing roll mounted on an end of the rotational spindle is attached to a lateral surface of the wheel head facing the grinding wheel. However, this truing apparatus is required to rotate the rotational spindle at such a high speed and gives rise to a problem that the apparatus becomes a large scale to increase the cost. Further, since the grinding wheel has to be retracted through a long distance at the time of a truing operation, the moving stroke of the grinding wheel is elongated thereby to enlarge the grinding machine. In addition, the high speed rotation of the rotational spindle causes the truing apparatus to increase heat generation, and such heat is conducted to the work head and the bed. As a consequence, a thermal displacement is brought about, e.g., between the axis of the rotational spindle with the truing roll mounted thereon and the axis of the work spindle, so that an error may be involved in the distance between the grinding wheel surface which has to be trued with the truing roll and the axis of the work spindle.